Janus kinases (JAKs) are protein tyrosine kinases ubiquitously expressed in cells. JAKs are involved in membrane signalling events which are triggered by a variety of extracellular factors that interact with cell surface receptors. JAKs initiate the cytoplasmic signal transduction cascades of cytokine receptors that lack a protein tyrosine kinase domain. The signal transduction cascades are initiated after oligomerisation of surface receptors due to ligand binding. Cytoplasmic receptor-associated JAKs are then activated which subsequently phosphorylate tyrosine residues along the receptor chains. These phosphotyrosine residues are targets for a variety of SH2 domain-containing transducer proteins, such as the signal transducers and activators of transcription (STAT) proteins. After STAT binds to receptor chains, they are phosphorylated by the JAK proteins, dimerise and translocate into the nucleus. In the nucleus, STAT alter the expression of cytokine-regulated genes.
Mammalian JAK-2 belongs to a kinase family that include JAK-1, JAK-3 and TYK-2. JAK-1, JAK-2, and TYK-2 are ubiquitously expressed, while JAK-3 is predominantly expressed in hematopoietic cells. These kinases consist of approximately 1150 amino acids, with molecular weights of about 120 kDa to 130 kDa. The amino acid sequences of the JAK kinase family are characterised by the presence of highly conserved domains. These domains include the JAK homology (JH) domains, C-terminal domain (JH1) responsible for the tyrosine kinase function, the tyrosine kinase-like domain (JH2) that shows high similarity to functional kinases but does not possess any catalytic activity, and the N-terminal domain that spans JH7 to JH3) that is important for receptor association and non-catalytic activity. Although the function of the N-terminal domain is not well established, there is some evidence for a regulatory role on the JH1 domain, thus modulating catalytic activity.
The down-stream substrates of the JAK family of kinases include the signal transducer and activator of transcription (STAT) proteins. JAK/STAT signaling has been implicated in the mediation of many abnormal immune responses such as allergies, asthma, autoimmune diseases such as transplant rejection, rheumatoid arthritis, amyotrophic lateral sclerosis and multiple sclerosis as well as in solid and hematologic malignancies such as leukemias and lymphomas.
Signal transducer and activator of transcription (STAT) proteins are activated by JAK family kinases. Recent studies suggested the possibility of modulating the JAK/STAT signaling pathway by targeting JAK family kinases with specific inhibitors for the treatment of leukemia (see Sudbeck, et al, Clin. Cancer Res. 5: 1569-1582 (1999)). In animal models, TEL/JAK-2 fusion proteins induced myeloproliferative disorders. In hematopoietic cell lines, introduction of TEL/JAK2 resulted in activation of STAT1, STAT3, STAT5, and cytokine-independent growth.
The JAK/STAT pathway is involved in abnormal cell growth. STAT3, STAT5, JAK1 and JAK2 are constitutively activated in mouse T cell lymphoma characterized initially by LCK over-expression. In addition, IL-6-mediated STAT3 activation was blocked by inhibition of JAK, leading to sensitization of myeloma cells to apoptosis.
One particularly attractive target for small-molecule modulation, with respect to antiproliferative and antiangiogenic activity, is JAK-2. Accumulating evidence shows that constitutive activation of JAK/STAT pathway promotes cell growth, survival, differentiation, neoplastic transformation, and angiogenesis in response to growth factors, cytokines, and hormones. JAK-2 is also activated in a wide variety of human cancers such as prostrate, colon, ovarian, breast cancers, melanoma, leukemia and other haematopoietic malignancies. In addition, somatic point mutation of the JAK-2 gene has been identified to be highly associated with classic myeloproliferative disorders (MPD) and infrequently in other myeloid disorders. Constitutive activation of JAK-2 activity is also caused by chromosomal translocation in hematopoeitic malignancies, such as in TEL-JAK-2 which is primarily associated with T-ALL, and in PCM1-JAK-2 which is associated with B-ALL and CML. It has been shown that inhibition of the JAK/STAT pathway, and in particular inhibition of JAK-2 activity, results in anti-proliferative and pro-apoptotic effects largely due to inhibition of phosphorylation of STAT. Furthermore, inhibition of JAK-2 activity by pharmacological agents or by expression of dominant negative JAK-2 effectively block tumor growth and induce apoptosis by reducing the STAT phosphorylation in cell culture and human tumor xenografts in vivo. Therefore, the JAK/STAT signal transduction pathway is a well-validated target pathway for therapeutic intervention.
Accordingly, the identification of small-molecule compounds that specifically inhibit, regulate and/or modulate the signal transduction of kinases, particularly JAK-2, is desirable as a means to treat or prevent diseases and conditions associated with cancers. Thus, an object of this invention is the identification of JAK-2 inhibitors as new therapeutic agents to treat human diseases.